Direct coupling of a high-brightness illumination source to a microscope potentially maximizes the optical coupling efficiency. Traditionally, an arc lamp is directly coupled to a fluorescence microscope by adjusting a collimating lens to achieve Koehler illumination. However, more modern light sources use a non-adjustable collimating optic, for example, the LDGI X-Cite series, which couples light indirectly through a liquid light guide and homogenizer, as described for example in U.S. Pat. No. 7,016,566. This type of light source is much simpler to use since it removes the need for periodic re-alignment required for traditional direct-coupled light sources. However, for direct coupling with a non-adjustable adaptor, the uniformity at the objective plane of an illumination source is a challenge for both lamp and LED light sources. For example, electrode structures on LEDs may be used to spread the current over the emitting area. These electrodes block light emission, causing undesirable optical non-uniformity.
One way to improve the uniformity of illumination is to insert a diffuser, with an appropriate diffusing grit, or with a fly eye lens array into the optical path. Typical diffusers come in the form of ground glass plates (but could also be coated glass or holographic glass). An ideal diffuser would scatter the light in a Lambertian pattern with zero loss. The amount of scatter or diffusion is determined by the “grit.” Grit refers to the size of the sand particle used in sandblasting of the glass. In practice, the diffuser generally causes a significant decrease in the optical power coupled to the objective plane. This may not be an issue with powerful, high-intensity arc lamp sources. However, for LED sources, the power loss incurred may be unacceptable. Furthermore, a fly eye lens array is expensive, and is often not a cost-effective option for the majority of microscopy applications.
Thus, there is a need for improved or alternative microscopy illumination systems, particularly those based on solid-state light sources that address one or more of the above-mentioned problems.